Longitudinal intensity oscillations observed with TRACE: evidence of fine-scale structure

The aim of this paper is two-fold: to increase the number of examples of observed longitudinal oscillations in coronal loops and to find evidence of the small temporal and spatial scales of these loop oscillations. Increasing the number of observed longitudinal oscillations allows for improvement in the statistics of the measured parameters, providing more accurate values for numerical and theoretical models. Furthermore, the small temporal and spatial scales of these loop oscillations could give indication of a driving force, symptomatic of coupling with the global p-modes. We found evidence that individual loop strands of wide coronal loop footpoints oscillate independently for short time periods. These strands have a diameter of the order of a few Mm, and the timescales on which the oscillations exist are typically less than an hour. We suggest that this is indicative of the oscillating strands being driven by the leakage of the global 5 min p-modes up into the corona, as simulated by De Pontieu et al. (2005, ApJ, 624, L61). Additionally, we find 25 further examples, added to those of De Moortel et al. (2002a, Sol. Phys., 209, 89), of outwardly propagating slow MHD waves in coronal loop footpoints. The datasets are taken from JOP83, observed between April 21st 2003 and May 3rd 2003, in the TRACE 171 angstrom bandpass. The intensity oscillations travel outwards with a propagation speed of order v = 99.7 +/- 3.9 km s(-1) and they are of small amplitude, with variations of approximately 3.7 +/- 0.2% of the background intensity. These disturbances are only detected for short distances, around 8.3 +/- 0.6 Mm along the loops, and the main period of oscillation is around 300 s. A second peak of period was found at around 200 s, however no correlation with the presence of a sunspot was observed within this study. Using these measured parameters we have estimated the energy flux to be of order 313 +/- 26 erg cm(-2) s(-1).